Ultracapacitor with improved aging performance

ABSTRACT

An energy storage device such as an electric double layer capacitor has positive and negative electrodes, each including a blend of respective first and second activated carbon materials having distinct pore size distributions. The blend (mixture) of first and second activated carbon materials may be equal in each electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/164,855, filed on Jan. 27, 2014, which the benefit of priority under35 U.S.C. §119 of U.S. Application Ser. No. 61/895,054 filed on Oct. 24,2013, the entire content of which is hereby incorporated by reference.

FIELD

The present disclosure relates generally to activated carbon materialsand more specifically to electric double layer capacitors comprisingactivated carbon-based electrodes.

SUMMARY

According to one embodiment, an electric double layer capacitorcomprises a positive electrode and a negative electrode each comprisinga first activated carbon material and a second activated carbonmaterial. In embodiments, the first activated carbon material has a poresize distribution different than the second activated carbon materialand each of the first activated carbon material and the second activatedcarbon material have <0.15 cm³/g combined pore volume of any poreshaving a size of >2 nm.

In further embodiments, the first activated carbon material comprisespores having a size of ≦1 nm, which provide a combined pore volumeof >0.3 cm³/g; pores having a size from >1 nm to ≦2 nm, which provide acombined pore volume of ≧0.05 cm³/g; and <0.15 cm³/g combined porevolume of any pores having a size of >2 nm. The second activated carbonmaterial comprises pores having a size of ≦1 nm, which provide acombined pore volume of ≦0.3 cm³/g; pores having a size from >1 nm to ≦2nm, which provide a combined pore volume of ≧0.05 cm³/g; and <0.15 cm³/gcombined pore volume of any pores having a size of >2 nm.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from that description or recognized by practicing theinvention as described herein, including the detailed description andthe claims.

It is to be understood that both the foregoing general description andthe following detailed description present embodiments of thedisclosure, and are intended to provide an overview or framework forunderstanding the invention as it is claimed.

The accompanying drawings are included to provide a furtherunderstanding of the disclosure, and are incorporated into andconstitute a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustrating potential shifts in an EDLC due toFaradic reactions;

FIG. 2 is a schematic illustration of an example ultracapacitor;

FIG. 3 is a pore size distribution histogram for example activatedcarbon materials;

FIG. 4 is a plot of normalized capacitance versus time for various EDLCconfigurations; and

FIG. 5 is a series of polarization plots for various EDLCconfigurations.

DETAILED DESCRIPTION

Reference will now be made in greater detail to various embodiments ofthe subject matter of the present disclosure, some embodiments of whichare illustrated in the accompanying drawings. The same referencenumerals will be used throughout the drawings to refer to the same orsimilar parts.

Energy storage devices such as ultracapacitors may be used in a varietyof applications such as where a discrete power pulse is required.Example applications range from cell phones to hybrid vehicles.

Ultracapacitors typically comprise two porous electrodes that areisolated from electrical contact with each other by a porous dielectricseparator. The separator and the electrodes are impregnated with anelectrolytic solution, which allows ionic current to flow between theelectrodes while preventing electronic current from discharging thecell. Each electrode is typically in electrical contact with a currentcollector. The current collector, which can comprise a sheet or plate ofelectrically-conductive material (e.g., aluminum) can reduce ohmiclosses while providing physical support for the porous electrodematerial.

Energy storage is achieved by separating and storing electrical chargein the electrochemical double layers that are created at the interfacesbetween the electrodes and the electrolyte. Specifically, within anindividual ultracapacitor cell and under the influence of an appliedelectric potential, an ionic current flows due to the attraction ofanions in the electrolyte to the positive electrode and cations to thenegative electrode. Ionic charge can accumulate at each of the electrodesurfaces to create charge layers at the solid-liquid interfaces. Theaccumulated charge is held at the respective interfaces by oppositecharges in the solid electrode to generate an electrode potential.Generally, the potential increases as a linear function of the quantityof charged species (ions and radicals) stored at or on the electrode.

During discharge of the cell, a potential across the electrodes causesionic current to flow as anions are discharged from the surface of thepositive electrode and cations are discharged from the surface of thenegative electrode. Simultaneously, an electronic current can flowthrough an external circuit located between the current collectors. Theexternal circuit can be used to power electrical devices. Though it isan electrochemical device, no chemical reactions are typically involvedin the energy storage mechanism. The mechanism is reversible, whichallows the ultracapacitor to be charged and discharged many times.

The performance of electric double layer capacitors (EDLCs) comprisingcarbon-based electrodes can be intimately related to the properties ofthe carbon. Important characteristics of these devices are the energydensity and power density that they can provide. The total availableporosity and pore size distribution of the activated carbon can impactEDLC performance. Moreover, it has been commonly thought thatsignificant quantities of mesopores are needed for electrolyte ionaccess to the interior surfaces of the carbon material. In thisdisclosure, it is demonstrated that EDLCs comprising microporousactivated carbon that is tailored to the size of the respective positiveand negative ions in the electrolyte exhibit a high specific capacitance(or energy density) and superior resistance to capacitive aging thanEDLCs using conventional carbon-based electrodes. This advantage isattributable to the customized pore size distribution of the carbonmaterials.

The activated carbon materials, which form the basis of the electrodes,can be made from natural or synthetic precursor materials. Naturalprecursor materials include coals, nut shells, and biomass. Syntheticprecursor materials typically include phenolic resins. With both naturaland synthetic precursors, activated carbon can be formed by carbonizingthe precursor and then activating the resulting carbon. The activationcan comprise physical (e.g., steam) or chemical (e.g., KOH) activationto increase the porosity and hence the surface area of the carbon.Carbon-based electrodes can include, in addition to activated carbon, aconductive carbon such as carbon black, and a binder such aspolytetrafluoroethylene (PTFE) or polyvinylidene fluoride (PVDF). Theactivated carbon-containing layer (carbon mat) is typically laminatedover a current collector to form the carbon-based electrode.

The choice of electrode materials directly affects the performance ofthe device, including the achievable energy density and power density.The energy density (E) of an EDLC is given by E=½CV², and the powerdensity (P) of an EDCL is given by P=V²/R, where C is the capacitance, Vis the device's operating voltage, and R is the equivalent seriesresistance (ESR) of the device.

In relation to the capacitance, a beneficial attribute is the ability tomaintain (or not significantly lose) capacitance over time as a resultof multiple charge-discharge cycles that accumulate with use. Aging ofthe carbon materials, such as by radical or ion trapping, can reduce theuseful life of ultracapacitors comprising activated carbon-basedelectrodes.

Accordingly, it would be an advantage to provide activated carbonmaterials and device architectures comprising activated carbon materialsas well as methods for making activated carbon materials having a highspecific capacitance that are resistant to aging. Such materials can beused to form carbon-based electrodes that enable efficient, long-lifeand high energy density devices.

Recently, with a goal of increasing the energy density and power densityof EDLC devices, engineered carbon materials have been developed toachieve higher capacitance. To achieve higher capacitance, activatedcarbon materials with high surface area (500-2500 m²/g) may ay be used.

A further approach to increasing the energy density and power density isto increase the capacitor's operating voltage. In this regard, aqueouselectrolytes have been used in EDLCs for lower voltage (<1V) operation,while organic electrolytes have been used for higher voltage (2.3-2.7 V)devices. However, to achieve even higher energy densities, there is aneed to increase the voltage envelop from conventional values of about2.7 V to around 3.0 V. Such an increase from 2.7 to 3.0 V will result ina 23% increase in the energy density.

Operation at higher voltages subjects the EDLC components to severaldifferent types of stresses that may lead to faster deterioration of thedevice. Such stresses include, for example, mechanical stresses on theelectrodes due to movement of charged ions back-and-forth into the poresof the activated carbon, and chemical stresses due to generation ofby-product gases as well as chemical degradation. The chemical stressesare in most part due to Faradic charge transfer processes in the cell.

Insomuch as higher energy densities and higher power densities may bepursued in next generation EDLCs via operation at high applied voltages,it will be desirable to provide activated carbon to minimize unwantedFaradaic reactions, particularly at the higher potentials. Such devicesmay be achieved in accordance with various embodiments by engineeringthe pore size distribution and impurity content of the activated carbonused to form the electrodes.

These Faradic charge transfer processes manifest as oxidation andreduction reactions at each of the positive and negative electrode ofthe EDLC. Such irreversible Faradic charge transfer processes may causethe potential of the electrodes to shift unfavorably. This is shownschematically in FIG. 1, which illustrates the potential responsethrough successive charge (C) and discharge (D) cycles for threedifferent examples. In the left-most pane, there are no Faradicreactions. In the middle pane, which corresponds to Faradic oxidation(FO) reactions at the positive electrode, the potential of the positiveelectrode will shift at the oxidation potential (OP) resulting in a netnegative potential shift of the positive electrode as well as a netnegative shift of the negative electrode (illustrated by thesingle-headed arrows) toward the reduction limit of the of theelectrolyte potential window. In a similar vein, in the right-hand pane,Faradic reduction (FR) at the negative electrode is illustrated at thereduction potential (RP), which causes a net positive potential shift inthe electrodes, including a shift of the positive electrode toward theoxidation limit of the electrolyte potential window. Either of theseoxidation or reduction mechanisms can adversely affect the performanceof an associated device.

The surface area, surface functional groups and the porosity and poresize distribution of the activated carbon can affect the performance ofthe cell. FIG. 2 is a schematic illustration of an exampleultracapacitor 10, which includes the blended electrode architecturedisclosed herein. Ultracapacitor 10 includes an enclosing body 12, apair of current collectors 22, 24, a first carbon mat 14 and a secondcarbon mat 16 each formed over one of the current collectors, and aporous separator layer 18. Electrical leads 26, 28 can be connected torespective current collectors 22, 24 to provide electrical contact to anexternal device. Carbon mats 14, 16 each comprise a mixture of at leasttwo porous activated carbon materials as disclosed herein. A liquidelectrolyte 20 is contained within the enclosing body 12 andincorporated throughout the porosity of both the porous separator layerand each of the porous electrodes. In embodiments, individualultracapacitor cells can be stacked (e.g., in series) to increase theoverall operating voltage. Ultracapacitors can have a jelly roll design,prismatic design, honeycomb design, or other suitable configuration.

The enclosing body 12 can be any known enclosure means commonly-usedwith ultracapacitors. The current collectors 22, 24 generally comprisean electrically-conductive material such as a metal, and commonly aremade of aluminum due to its electrical conductivity and relative cost.For example, current collectors 22, 24 may be thin sheets of aluminumfoil.

Porous separator 18 electronically insulates the electrodes from eachother while allowing ion diffusion. The porous separator can be made ofa dielectric material such as cellulosic materials, glass, and inorganicor organic polymers such as polypropylene, polyesters or polyolefins. Inembodiments, a separator layer thickness can range from about 0.5 mil to10 mils.

The electrolyte 20 serves as a promoter of ion conductivity, as a sourceof ions, and may serve as a binder for the carbon. The electrolytetypically comprises a salt dissolved in a suitable solvent. Suitableelectrolyte salts include quaternary ammonium salts such as thosedisclosed in commonly-owned U.S. Patent Application Publication Nos.2013/0075647, 2013/0207019 and U.S. patent application Ser. No.13/909,645, the disclosures of which are incorporated herein byreference. An example quaternary ammonium salt is tetraethylammoniumtetrafluoroborate ((Et)₄NBF₄).

Example solvents for the electrolyte include but are not limited tonitrites such as acetonitrile, acrylonitrile and propionitrile;sulfoxides such as dimethyl, diethyl, ethyl methyl and benzylmethylsulfoxide; amides such as dimethyl formamide and pyrrolidones such asN-methylpyrrolidone. In embodiments, the electrolyte includes a polaraprotic organic solvent such as a cyclic ester, chain carbonate, cycliccarbonate, chain ether and/or cyclic ether solvent. Example cyclicesters and chain carbonates have from 3 to 8 carbon atoms, and in thecase of the cyclic esters include β-butyro-lactone, γ-butyrolactone,γ-valerolactone and δ-valerolactone. Examples of the chain carbonatesinclude dimethyl carbonate, diethyl carbonate, dipropyl carbonate,methyl ethyl carbonate, methyl propyl carbonate and ethyl propylcarbonate. Cyclic carbonates can have from 5 to 8 carbon atoms, andexamples include 1,2-butylene carbonate, 2,3-butylene carbonate,1,2-pentene carbonate, 2,3-pentene carbonate and propylene carbonate.Chain ethers can have 4 to 8 carbon atoms. Example chain ethers includedimethoxyethane, diethoxyethane, methoxyethoxyethane, dibutoxyethane,dimethoxypropane, diethoxypropane and methoxyethoxypropane. Cyclicethers can have from 3 to 8 carbon atoms. Example cyclic ethers includetetrahydrofuran, 2-methyl-tetrahydrofuran, 1,3-dioxolan, 1,2-dioxolan,2-methyldioxolan and 4-methyl-dioxolan.

As examples, an assembled EDLC can comprise an organic liquidelectrolyte such as tetraethylammonium tetrafluoroborate (TEA-TFB) ortriethylmethylammonium tetrafluoroborate (TEMA-TFB) dissolved in anaprotic solvent such as acetonitrile.

With ionic salts such as tetraethylammonium tetrafluoroborate, thetetraethylammonium cation is larger than the tetrafluoroborate anion.Without wishing to be bound by theory, the size of the (Et)₄N⁺ cation isestimated be about 0.68 nm, while the size of the BF₄ ⁻ anion isestimated to be about 0.48 nm.

Conventional approaches to the design of carbon-based electrodestypically involve maximizing the internal volume of the carbon material,which maximizes the achievable energy density. Specifically, theseapproaches lead to a predominance of smaller pores which yield a highersurface area per unit volume and thus a higher capacitance. Smallerpores, however, may inhibit the access and adsorption of larger ions.Further, aging-associated deposition of decomposition products from theelectrolyte may inhibit ion movement or cause ion trapping, which canyield to an undesired attenuation in the capacitance over time and/orcycling of the ultracapacitor.

According to various embodiments, an energy storage device comprises apositive electrode and a negative electrode each comprising a blend of afirst activated carbon material and a second activated carbon material.In embodiments, each blended electrode comprises a physical mixture of afirst activated carbon material and a second activated carbon material.The first activated carbon material has a pore size distributiondifferent than the second activated carbon material and each of thefirst activated carbon material and the second activated carbon materialhave <0.15 cm³/g combined pore volume of any pores having a size of >2nm.

In related embodiments, the first activated carbon material comprisespores having a size of ≦1 nm, which provide a combined pore volumeof >0.3 cm³/g; pores having a size from >1 nm to ≦2 nm, which provide acombined pore volume of ≧0.05 cm³/g; and <0.15 cm³/g combined porevolume of any pores having a size of >2 nm; while the second activatedcarbon material comprises pores having a size of ≦1 nm, which provide acombined pore volume of ≦0.3 cm³/g; pores having a size from >1 nm to ≦2nm, which provide a combined pore volume of ≧0.05 cm³/g; and <0.15 cm³/gcombined pore volume of any pores having a size of >2 nm. For the sakeof clarity, activated carbon pore sizes may be any rational numberwithin the recited range. Thus, pores having a size from >1 nm to ≦2 nmmay include 1.01, 1.115, 1.6, 1.99 and 2.0 nm.

The positive electrode, in addition to including the first activatedcarbon material, includes up to an equal amount (e.g., wt. %) of thesecond activated carbon material. Likewise, the negative electrode, inaddition to including the second activated carbon material, includes upto an equal amount of the first activated carbon material.

With respect to the activated carbon content, the positive electrode mayinclude 50-75% first activated carbon and 25-50% second activatedcarbon, e.g., a ratio (wt. %/wt. %) of first activated carbon to secondactivated carbon in the positive electrode may be 50:50, 55:45, 60:40,65:35, 70:30 or 75:25. The negative electrode may include 25-50 wt. %first activated carbon and 50-75 wt. % second activated carbon, e.g., aratio (wt. %/wt. %) of first activated carbon to second activated carbonin the negative electrode may be 50:50, 45:55, 40:60, 35:65, 30:70 or25:75.

The first and second activated carbon materials can comprise micro-,meso- and/or macroscale porosity. As defined herein, microscale poreshave a pore size of 2 nm or less (and ultramicropores have a pore sizeof 1 nm or less). Mesoscale pores have a pore size ranging from 2 to 50nm. Macroscale pores have a pore size greater than 50 nm. In anembodiment, the activated carbons, e.g., first and second activatedcarbons, comprise a majority of microscale pores. The term “microporouscarbon” and variants thereof means an activated carbon having a majority(i.e., at least 50%) of microscale pores. Thus, a microporous, activatedcarbon material can comprise greater than 50, 55, 60, 65, 70, 75, 80,85, 90 or 95% microporosity. Energy storage devices disclosed herein mayinclude activated carbon that consists or consists essentially ofmicroporous carbon.

The pore size distribution of the activated carbon may be characterizedas having a unimodal, bimodal or multi-modal pore size distribution. Theultramicropores can comprise 0.2 cm³/g or more (e.g., 0.2, 0.25, 0.3,0.35 or 0.4 cm³/g or more) of the total pore volume. Pores having a poresize (d) in the range of 1<d<2 nm can comprise 0.05 cm³/g or more (e.g.,at least 0.1, 0.15, 0.2 or 0.25 cm³/g) of the total pore volume. Ifpresent, any pores having a pore size greater than 2 nm, which mayinclude mesopores and/or macropores, can comprise 0.15 cm³/g or less(e.g., less than 0.1 or 0.05 cm³/g) of the total pore volume.

According to embodiments, a carbon-based electrode for an EDLC comprisesfirst and second activated carbon materials each having a total porositygreater than about 0.4 cm³/g (e.g., greater than 0.4, 0.45, 0.5, 0.55,0.6, 0.65 or 0.7 cm³/g). In embodiments, the portion of the total porevolume resulting from micropores (d≦2 nm) can be about 90% or greater(e.g., at least 90, 92, 94, 96, 98 or 99%) and the portion of the totalpore volume resulting from ultramicropores (d≦1 nm) can be about 40% orgreater (e.g., at least 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90 or95%).

In example embodiments, a positive electrode comprises a first activatedcarbon material comprising pores having a size of ≦1 nm, which provide acombined pore volume of >0.3 cm³/g, pores having a size of >1 nm to ≦2nm, which provide a combined pore volume of ≧0.05 cm³/g, and <0.15 cm³/gcombined pore volume of any pores having a size of >2 nm. A negativeelectrode includes a second activated carbon material comprising poreshaving a size of ≦1 nm, which provide a combined pore volume of ≦0.3cm³/g, pores having a size of >1 nm to ≦2 nm, which provide a combinedpore volume of ≧0.05 cm³/g, and <0.15 cm³/g combined pore volume of anypores having a size of >2 nm.

The first activated carbon may, for example, comprise pores having asize of ≦1 nm, which provide a combined pore volume of >0.3 to 0.5cm³/g. Such activated carbon may have pores having a size of >1 nm to ≦2nm, which provide a combined pore volume of ≧0.2 cm³/g (e.g., 0.2 to 0.3cm³/g), and <0.1 or <0.05 cm³/g combined pore volume of any pores havinga size of >2 nm.

The second activated carbon may, for example, comprise pores having asize of ≦1 nm, which provide a combined pore volume of 0.2 to 0.3 cm³/g.Such activated carbon may have pores having a size of >1 nm to ≦2 nm,which provide a combined pore volume of ≧0.2 cm³/g (e.g., 0.2 to 0.3cm³/g), and <0.1 or <0.05 cm³/g combined pore volume of any pores havinga size of >2 nm.

A histogram of the respective pore size distributions for example firstand second activated carbons is shown in FIG. 3. In the illustratedexample, first activated carbon includes 0.45 cm³/g pore volume in poresless than ≦1 nm, 0.21 cm³/g pore volume in pores between >1 nm and ≦2nm, and 0.02 cm³/g pore volume in pores >2 nm. The second activatedcarbon is characterized by 0.27 cm³/g pore volume in pores less than ≦1nm range, 0.28 cm³/g pore volume in pores between >1 nm and ≦2 nm, and0.05 cm³/g pore volume in pores >2 nm.

In embodiments, an EDLC comprises a first activated carbon having acombined pore volume associated with ultramicropores that is greaterthan the corresponding combined pore volume of ultramicropores in thesecond activated carbon. In embodiments, the first activated carbon mayhave a combined pore volume associated with pores having a size of >1 nmto ≦2 nm that is less than the corresponding combined pore volume ofsuch size pores for the second activated carbon. In a still furtherexample EDLC, the first activated carbon may have a combined pore volumeassociated with any pores having a size of >2 nm that is less than thecombined pore volume of pores of such sized pores for the secondactivated carbon.

In the disclosed configurations, the carbon-based electrode thatinteracts with the smaller anion is engineered to comprise a greaterproportion of ultramicropores, while the carbon-based electrode thatinteracts with the larger cation is engineered to have a larger averagepore size. The blended carbon electrode assembly allows positive andnegative ions to easily move in and out of the pores of the respectivecarbon electrodes, which minimizes capacitance fade while maintainingexcellent performance.

In embodiments, the activated carbon can be characterized by a highsurface area. A carbon-based electrode for an EDLC can include carbonhaving a specific surface area greater than about 300 m²/g, i.e.,greater than 300, 350, 400, 500 or 1000 m²/g. Further, the activatedcarbon can have a specific surface area less than 2500 m²/g, i.e., lessthan 2500, 2000, 1500, 1200 or 1000 m²/g.

The activated carbon used to form the carbon-based electrodes can bederived from a variety of different carbon precursor materials. Examplecarbon precursor materials and associated methods of forming activatedcarbon are disclosed in commonly-owned U.S. patent application Ser. Nos.12/335,044, 12/335,078, 12/788,478 and 12/970,073, the entire contentsof which are hereby incorporated by reference.

In an example method, a carbon precursor material can be heated at atemperature effective to first carbonize the precursor material. Examplecarbonization temperatures are greater than about 450° C. (e.g., atleast 450, 500, 550, 600, 650, 700, 750, 800, 850 or 900° C.). An inertor reducing atmosphere can be used during carbonization of the carbonprecursor. Example gases and gas mixtures include one or more ofhydrogen, nitrogen, ammonia, helium and argon. The carbonized materialcan be activated.

Physical or chemical activation processes may be used to produceactivated carbon. In a physical activation process, raw material orcarbonized material is exposed to typically oxidizing conditions (oxygenor steam) at elevated temperatures (e.g., greater than 250° C.).Chemical activation on the other hand involves impregnating raw orcarbonized material with an activating agent, and then heating theimpregnated carbon to a temperature typically in the range of 400-900°C. Chemical activating agents include alkali hydroxides or chlorides(e.g., NaOH, KOH, NaCl, KCl), phosphoric acid, or other suitable saltssuch as CaCl₂ or ZnCl₂.

In the various examples disclosed herein, a KOH-activated carbon derivedfrom wheat flour was used to prepare the first activated carbon materialand a commercially-available steam-activated carbon derived from coconutshells was used to prepare the second activated carbon material.Following chemical activation, the activated carbon can be washed toremove inorganic compounds and any chemical species derived fromreactions involving the activating agent. Whether produced by steam orby chemical activation, the activated carbon can be dried and optionallyground to produce material comprising a substantially homogeneousdistribution of porosity.

The performance (energy and power density) of an ultracapacitor dependslargely on the properties of the activated carbon that makes up theelectrodes. The activated carbon materials disclosed herein can be usedto form carbon-based electrodes for economically viable, high power,high energy density devices. The properties of the activated carbon, inturn, can be gauged by evaluating the surface area, porosity and poresize distribution of the material, as well as by evaluating theelectrical properties of a resulting ultracapacitor. Relevant electricalproperties include the area-specific resistance, and the specificcapacitance.

Whether the activated carbon material is predominately ultra-microporousor microporous, the presence of oxygen in the carbon, especially in theform of oxygen-containing surface functionalities, can adversely affectthe properties of energy storage devices that comprise electrodes madefrom the activated carbon. For example, the presence ofoxygen-containing surface functionalities can give rise topseudocapacitance, increase the self-discharge or leakage rate, causedecomposition of the electrolyte, and/or cause a long term increase inresistance and deterioration of capacitance. Oxygen functionalities canbe introduced during the carbonization and activation steps, where theactivating agent (e.g., steam or KOH) serves as an oxidation agent.

As a result of the potentially deleterious effects of incorporatedoxygen, it can be advantageous to control and preferably minimize theoxygen content in activated carbon for use in energy storage devicessuch as EDLCs.

In embodiments, activated carbon whether formed by physical or chemicalactivation, is subjected to a refining step wherein the activated carbonis heated in an inert or reducing environment to a temperature rangingfrom, for example, about 450-1000° C., e.g., 900° C., and for a periodof, for example, about 0.5-10 hours. Preferably, the environment duringthe refining step is substantially free of oxygen. The refining stepreduces the oxygen content in the activated carbon. One method to reduceoxygen content is to refine (heat) the activated carbon in an inertenvironment (such as nitrogen, helium, argon, etc.) or in a reducingenvironment (such as hydrogen, forming gas, carbon monoxide, etc.).Example refining experiments were conducted in a retort furnace (CMFurnaces, Model 1212FL) purged with nitrogen.

In embodiments, the total oxygen content of the activated carbon is atmost 1.5 wt. %. By total oxygen content is meant the sum of all atomicand molecular oxygen in the carbon, including oxygen inoxygen-containing functional groups in and/or on the carbon.

In embodiments, the total oxygen content of carbon black used to formcarbon-based electrodes can be decreased in a parallel approach. Forinstance, prior to mixing activated carbon with carbon black and binder,the activated carbon and the carbon black can be heat-treated todecrease the oxygen content. Such heat-treatments of the activatedcarbon and the carbon black can be carried out separately, or in aunified process by mixing the activated carbon and the carbon black, andheating the mixture prior to combining the mixture with a binder. Inembodiments, the total oxygen content of carbon black is at most 1.5 wt.%.

Once formed and optionally refined, the activated carbon can beincorporated into a carbon-based electrode. In a typical electric doublelayer capacitor (EDLC), a pair of carbon-based electrodes is separatedby a porous separator and the electrode/separator/electrode stack isinfiltrated with a liquid organic or inorganic electrolyte. Theelectrodes may comprise activated carbon that has been mixed with otheradditives (e.g., binders) and compacted into a carbon mat and laminatedto a conductive metal current collector backing.

By way of example, a carbon mat having a thickness in the range of about100-300 micrometers can be prepared by rolling and pressing a powdermixture comprising 60-90 wt. % activated carbon, 5-20 wt. % carbon blackand 5-20 wt. % PTFE. The carbon black component of the carbon mat can bedivided between a first activated carbon and a second activated carbon,where the proportion of first and second activated carbon depends onwhether the carbon-based electrode will be formed into a positiveelectrode or a negative electrode. Carbon sheets can be stamped orotherwise patterned from the carbon mat and laminated to a conductivecurrent collector to form a carbon-based electrode. The carbon-basedelectrode can be incorporated into an energy storage device.

According to an embodiment, an electrochemical cell comprises a positiveelectrode comprising a first activated carbon material and a secondactivated carbon material, a negative electrode comprising a firstactivated carbon material and a second activated carbon material, aporous separator, and a pair of electrically conductive currentcollectors, wherein the porous separator is disposed between thepositive electrode and the negative electrode, and the positive andnegative electrodes are each in electrical contact with a respectivecurrent collector. In embodiments, the first activated carbon representsat least 50% of the activated carbon content in the positive electrode,and the second activated carbon represents at least 50% of the activatedcarbon content in the negative electrode.

In addition to adjusting the pore size distribution, Applicants havefound that the initial capacitance of an ultracapacitor comprisingblended carbon-based electrodes can be increased by increasing thethickness of the negative electrode relative to the positive electrode.In embodiments, the negative electrode thickness can be 5, 10, 20, 30,40, 50, 60, 70, 80, 90 or 100% greater than the positive electrodethickness.

EXAMPLES

The disclosure will be further clarified by the following examples.

Example 1

Comparative EDLCs were fabricated by incorporating only a secondactivated carbon material into each electrode. Particles of the secondactivated carbon were mixed with PTFE (DuPont 601A) and carbon black(Cabot BP2000) in the ratio (by weight) of 85:10:5 using a mediumintensity Lab Master mixer at room temperature. Approximately 5% IPA byweight was added to the mixture to aid in fibrillation. The fibrillationprocess was conducted using a twin screw auger to obtain granules, whichwere then broken in a Fitz Mill to achieve a fine powder.

This powder mixture was calendared by passing it through a series ofpressure rollers at 100° C. to form a 100 μm thick carbon mat. Two suchcarbon mats were laminated onto opposite sides of a conductive carbonink-coated current collector to form a carbon-based electrode. Thecurrent collector was a 25 μm thick aluminum foil provided with a 5 μmthick coating of conductive carbon ink (DAG EB012 from Henkel, formerlyAcheson).

Two of such electrodes, separated by porous separator paper TF4030(Nippon Kodoshi Corporation) were wound into a jelly roll andpackaged/sealed in an aluminum container. The assembled cell was vacuumdried at 130° C. for 48 h before being filled with a 1.2 M solution ofTEA-TFB (tetraethyl ammonium tetrafluoroborate) electrolyte inacetonitrile.

The cell was heated and then subjected to constant voltage stress testat 2.7 V. The initial cell capacitance was 395 F, with a cell ESR of1.74 mΩ. A plot of normalized capacitance as function of aging time isshown in FIG. 4. The time to 80% normalized capacitance was 910 hrs.

Example 2

A comparative cell was fabricated using the procedure of Example 1,except only a first activated carbon material was used to form thepositive electrode and only a second activated carbon material was usedto form the negative electrode.

The cell was heated and then subjected to constant voltage stress testat 2.7 V. The initial capacitance was 452 F. The cell ESR was 2.2 mΩ. Aplot of normalized capacitance as function of aging time is shown inFIG. 4. The time to 80% normalized capacitance for the device was 260hrs.

Compared to Example 1, the tuned geometry of the instant exampleexhibits a slightly higher initial capacitance. However, while theinitial capacitance fade is less for the tuned cell, without wishing tobe bound by theory, the higher rate of capacitance degradation afterabout 200 h is believed to be due to the fact that in the currentconfiguration, the TEA-TFB electrolyte unfavorably positions theelectrode potentials outside the electrolyte voltage window into adecomposition region. Without wishing to be bound by theory, assummarized schematically in FIG. 5, which is a graphic showing thepolarization potentials for differently-configured positive and negativeelectrodes, the positive electrode potential in the tuned cell is closeto the acetonitrile oxidation potential (AOP) of about 1.7V or greater.Also shown is the acetonitrile reduction potential (ARP) of about −1.7Vor less. The electrode potentials are plotted with reference to a normalhydrogen electrode (NHE).

Example 3

To address the high capacitive degradation associated with the tunedcell configuration of Example 2, an EDLC was fabricated using a blend offirst and second activated carbon materials in each electrode. In thecurrent example, the positive electrode was fabricated using a blend of75% first activated carbon and 25% second activated carbon. The negativeelectrode was fabricated using a blend of 25% first activated carbon and75% second activated carbon.

The cell was heated and then subjected to constant voltage stress testat 2.7 V. The initial capacitance for the cell was 454 F, with a cellESR of 2.5 mΩ. As shown in the FIG. 4 plots of normalized capacitanceversus time, the time to 80% normalized capacitance for the device was510 hrs.

Compared to Example 2, the blended electrode design decreased thecapacitance degradation by favorably moving the electrode potentialsinside the electrolyte voltage window.

Example 4

A further EDLC was fabricated using blended electrodes. To simplify themanufacturing in the current example, each of the positive and negativeelectrodes was fabricated using a 50:50 blend of first and secondactivated carbon materials.

The cell was heated and then subjected to constant voltage stress testat 2.7 V. The initial capacitance for the cell was 456 F. The cell ESRwas 2.2 mΩ. Referring to the FIG. 4 data, the time to 80% normalizedcapacitance for the device was 1000 hrs.

The capacitance is similar to that of the tuned cell configuration;however, the degradation of the blended electrode device is improvedsignificantly due to favorable positioning of the electrode potentialsinside the electrolyte voltage window, and a reduction of cationtrapping.

Example 5

Further comparative EDLCs were fabricated by incorporating a firstactivated carbon material into each electrode using the processdescribed in Example 1. Following the 2.7V stress test, the cell had aninitial capacitance of 491 F, and an ESR of 3.4 mΩ. As shown in FIG. 4,the time to 80% normalized capacitance was 450 hrs.

Although the vast majority of open pores in the first activated carboncan contribute to the measured surface area, without wishing to be boundby theory, it is believed that not all the pores are electrochemicallyaccessible. In the present example, rapid degradation of the capacitancewas observed in the first 200 hours.

Example 6

The tuned cell configuration of Example 2 was repeated using a 1.2 Msolution of TEMA-TFB (triethylmethyl ammonium tetrafluoroborate) inacetonitrile in place of the 1.2M TEA-TFB (tetraethyl ammoniumtetrafluoroborate)-based electrolyte.

The cell was conditioned and then subjected to constant voltage stresstest at 2.7 V. The initial capacitance was 472 F, and the cell ESR was2.2 mΩ. The time to 80% normalized capacitance was 900 hrs. Thesubstitution of TEMA-TFB for TEA-TFB improves the degradation behaviorof the device, though TEMA-TFB is typically more expensive.

Example 7

A commercially-available EDLC (Maxwell—BCAP2000 P270 K04), which is a2000 F rated 2.7 V device containing TEA-TFB electrolyte, was stresstested at 3.0 V and 65° C. The initial capacitance was 2118 F, and theESR was 0.5 mΩ. The time to 80% normalized capacitance was 300 hrs.

Example 8

The configuration of Example 2 was repeated, and the cell wasconditioned and then subjected to constant voltage stress test at 3.0 V.The initial capacitance was 2911 F, and the ESR was 0.66 mΩ. The time to80% normalized capacitance was 185 hrs.

Example 9

The configuration of Example 6 was repeated, and the cell wasconditioned and then subjected to constant voltage stress test at 3.0 V.The initial capacitance was 2867 F, and the ESR was 0.46 mΩ. The time to80% normalized capacitance was 285 hrs. This example may be compared toExample 8. By comparing the time to 80% normalized capacitance, it isevident that TEMA-TFB electrolyte provides greater long-term stabilitythan TEA-TFB in a tuned cell configuration.

Results from Example 1-9 are summarized in Table 1, where the firstactivated carbon is abbreviated C1 and the second activated carbon isabbreviated C2. In Table 1, Examples 1, 2 and 5-9 are comparative.

TABLE 1 Example EDLC configurations. Pos. Neg. Cap ESR Time to 80% Ex.Electrode Electrode Electrolyte [F] [mΩ] Cap [hr] 1 100% C2 100% C2TEA-TBF 395.2 1.7 910 (2.7 V) 2 100% C1 100% C2 TEA-TBF 451.5 2.2 260(2.7 V) 3 75% C1 + 25% C2 25% C1 + 75% C2 TEA-TBF 454.4 2.5 510 (2.7 V)4 50% C1 + 50% C2 50% C1 + 50% C2 TEA-TBF 456.2 2.2 1000 (2.7 V)  5 100%C1 100% C1 TEA-TBF 490.8 3.4 450 (2.7 V) 6 100% C1 100% C2 TEMA-TBF472.0 2.2 900 (2.7 V) 7 100% C2 100% C2 TEA-TBF 2118.2 0.5 300 (3.0 V) 8100% C1 100% C2 TEA-TBF 2911.3 0.7 185 (3.0 V) 9 100% C1 100% C2TEMA-TBF 2866.7 0.5 285 (3.0 V)

This disclosure provides electric double layer capacitors havingpositive and negative electrodes that each comprise a blend of first andsecond microporous activated carbon materials. The first and secondactivated carbon materials may be homogeneously mixed or provided asdiscrete layers in a composite electrode. Such structures provide highspecific capacitance as well as improved long term stability. Methodsfor making such activated carbon materials are also disclosed.

As used herein, the singular forms “a,” “an” and “the” include pluralreferents unless the context clearly dictates otherwise. Thus, forexample, reference to a “carbon material” includes examples having twoor more such “carbon materials” unless the context clearly indicatesotherwise.

Ranges can be expressed herein as from “about” one particular value,and/or to “about” another particular value. When such a range isexpressed, examples include from the one particular value and/or to theother particular value. Similarly, when values are expressed asapproximations, by use of the antecedent “about,” it will be understoodthat the particular value forms another aspect. It will be furtherunderstood that the endpoints of each of the ranges are significant bothin relation to the other endpoint, and independently of the otherendpoint.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is no way intended thatany particular order be inferred.

It is also noted that recitations herein refer to a component being“configured” or “adapted to” function in a particular way. In thisrespect, such a component is “configured” or “adapted to” embody aparticular property, or function in a particular manner, where suchrecitations are structural recitations as opposed to recitations ofintended use. More specifically, the references herein to the manner inwhich a component is “configured” or “adapted to” denotes an existingphysical condition of the component and, as such, is to be taken as adefinite recitation of the structural characteristics of the component.

While various features, elements or steps of particular embodiments maybe disclosed using the transitional phrase “comprising,” it is to beunderstood that alternative embodiments, including those that may bedescribed using the transitional phrases “consisting” or “consistingessentially of,” are implied. Thus, for example, implied alternativeembodiments to a carbon mat that comprises activated carbon, carbonblack and a binder include embodiments where a carbon mat consistsactivated carbon, carbon black and a binder and embodiments where acarbon mat consists essentially of activated carbon, carbon black and abinder.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the present inventionwithout departing from the its spirit and scope. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments incorporating the spirit and substance of the disclosure mayoccur to persons skilled in the art, the invention should be construedto include everything within the scope of the appended claims and theirequivalents.

What is claimed is:
 1. An energy storage device comprising: a positiveelectrode and a negative electrode each comprising a first activatedcarbon material and a second activated carbon material; wherein thefirst activated carbon material comprises pores having a size of ≦1 nm,which provide a combined pore volume of >0.3 cm³/g; the second activatedcarbon material comprises pores having a size of ≦1 nm, which provide acombined pore volume of ≦0.3 cm³/g.
 2. The energy storage deviceaccording to claim 1, wherein the first activated carbon materialcomprises pores having a size of ≦1 nm, which provide a combined porevolume of >0.3 cm³/g; and pores having a size from >1 nm to ≦2 nm, whichprovide a combined pore volume of ≧0.05 cm³/g; and the second activatedcarbon material comprises pores having a size of ≦1 nm, which provide acombined pore volume of ≦0.3 cm³/g; and pores having a size from >1 nmto ≦2 nm, which provide a combined pore volume of ≧0.05 cm³/g.
 3. Theenergy storage device according to claim 1, wherein the positiveelectrode and the negative electrode each comprise a mixture of thefirst activated carbon material, the second activated carbon material, aconductive carbon, and a binder.
 4. The energy storage device accordingto claim 1, wherein the activated carbon content in the positiveelectrode is 50-75 wt. % first activated carbon material and 25-50 wt. %second activated carbon material, and the activated carbon content inthe negative electrode is 25-50 wt. % first activated carbon materialand 50-75 wt. % second activated carbon material.
 5. The energy storagedevice according to claim 1, wherein the activated carbon content ineach of the positive electrode and the negative electrode is 50 wt. %first activated carbon material and 50 wt. % second activated carbonmaterial.
 6. The energy storage device according to claim 1, wherein thefirst activated carbon material comprises pores having a size of ≦1 nm,which provide a combined pore volume of >0.3 to 0.5 cm³/g, and thesecond activated carbon material comprises pores having a size of ≦1 nm,which provide a combined pore volume of 0.2 to 0.3 cm³/g.
 7. The energystorage device according to claim 1, wherein the first activated carbonmaterial comprises pores having a size of >1 nm to ≦2 nm, which providea combined pore volume of ≧0.2 cm³/g; and the second activated carbonmaterial comprises pores having a size of >1 nm to ≦2 nm, which providea combined pore volume of ≧0.2 cm³/g.
 8. The energy storage deviceaccording to claim 6, wherein the combined pore volume of the poreshaving a size of >1 nm to ≦2 nm in the first activated carbon materialis less than the combined pore volume of the pores having a size of >1nm to ≦2 nm in the second activated carbon material.
 9. The energystorage device according to claim 1, wherein the first activated carbonmaterial comprises pores having a size of >2 nm, which provide acombined pore volume of <0.15 cm³/g; and the second activated carbonmaterial comprises pores having a size of >2 nm, which provide acombined pore volume of <0.15 cm³/g.
 10. The energy storage deviceaccording to claim 1, wherein the first activated carbon materialcomprises pores having a size of >2 nm, which provide a combined porevolume of <0.05 cm³/g; and the second activated carbon materialcomprises pores having a size of >2 nm, which provide a combined porevolume of <0.05 cm³/g.
 11. The energy storage device according to claim1, wherein a total pore volume of the first activated carbon material isgreater than 0.5 cm³/g and a total pore volume of the second activatedcarbon material is greater than 0.5 cm³/g.
 12. The energy storage deviceaccording to claim 1, wherein the device is an ultracapacitor.
 13. Theenergy storage device according to claim 1, further comprising anelectrolyte solution of tetraethylammonium tetrafluoroborate dissolvedin an aprotic solvent.